A floating roof storage tank generally has a solid cylindrical outer wall covered by a solid sloped or dome-shaped roof. A floating roof is held within the volume defined by the outer wall and roof. The floating roof extends over the liquid contents held within the volume, and forms a vapor seal around the internal circumference of the cylindrical wall. The height within the tank at which the floating roof is positioned varies according to the amount of liquid being stored within the tank at any given time.
Floating roof storage tanks generally are used to store flammable liquids, such as gasoline. A vapor space is formed inside the tank, between the floating roof covering the gasoline and the outer tank roof. Vapors emitted from the internal volume of the storage tank are collected in the vapor space to prevent significant amounts of vapor from being expelled to the atmosphere outside the tank. Environmental protection regulations restrict the amount of hydrocarbon vapors that may be released to the atmosphere. Air quality may be impacted adversely if hydrocarbon vapor emissions exceed permitted amounts.
Manual access to the internal volume of the tank is provided within the vapor space. One or more access ports or doorways are formed in the roof or outer wall of the tank, and a ladder or gangway may be supported within the vapor space. Persons may enter the vapor space to service the tank, the internal floating roof and any equipment housed inside the tank. Because of the hazards associated with confined space entry within the vapor space, particularly in flammable atmospheres, entry for inspection purposes is rarely permitted.
UL 913-1988 sets forth the standards that must be satisfied before equipment may be introduced into a floating roof storage tank or the vapor space above the floating roof in such tank. “Intrinsically safe” apparatus are protected against ignition by (1) limiting the maximum current and voltage that may be applied so that the energy available in a hazardous location is not capable of igniting the flammable mixture in such location; and (2) taking precautions against spark ignition and thermal ignition.
Special precautions are taken when introducing equipment into the internal volume of the floating roof storage tank when the tank volume holds a flammable liquid, such as gasoline. Measures are taken to prevent sparks and associated combustion of flammable vapors. Measures are also taken to minimize the amount of time the seal between the flammable liquid and the vapor space above the floating roof is broken to prevent substantial amounts of flammable vapors from being emitted into the vapor space.
Floating roof storage tanks are inspected at regular intervals to locate cracks, corrosion or other defects that might lead to tank failure. Environmental protection regulations specify the frequency and recommended procedures for inspecting tanks for structural integrity. One method involves introducing a remote controlled submersible vehicle into the tank while the tank remains in service. For example, U.S. Pat. No. 5,205,174 discloses a scavenger submersible vehicle that inspects the internal surfaces of a liquid filled tank using a video camera and/or ultrasound pulses. One or more umbilical hose(s) and wire harness(es) connect the remote-controlled vehicle to its power source and air or vacuum source, and further provide a link for transmitting navigation and inspection data from the vehicle to one or more computers located outside the tank.
Known hydraulic hoses formed from rubber or nylon reinforced thermoplastics are not “intrinsically safe” because excessive heat energy builds from friction as the hose surface contacts the pulley or the surfaces of the internal floating roof tank when the inspection vehicle is deployed into the tank. In addition, rubber materials may degrade in caustic chemical environments. Known PTFE hydraulic hoses can withstand caustic environments, but have not been found conductive enough to dissipate static charges. When such PTFE hoses are covered with a metallic braided outer sheath, such as stainless steel, to increase conductivity, they have also been found unsafe because the braided sheath scrapes or punctures the coatings over electrical cables bundled together with the hydraulic hoses in an umbilical connection. Moreover, the stainless braid can also scrape the surfaces of the internal floating roof, generating a spark or building up heat toward possible ignition in the vapor space.
The umbilical cord connection to a robotic inspection vehicle generally includes a bundle of hydraulic hoses and electrical cables wrapped in an outer sheath. To meet “intrinsically safe” standards, the sheath itself should be static dissipative, should not build up heat from frictional forces as the cord is deployed through a vapor space or hazardous environment, and should not abrade or damage the materials covering the hoses and cables within the sheath. Prior sheathing materials have not satisfied these rigorous requirements.